1. Field of the Invention
This invention relates to processes and devices for heating by utilizing of electrical resistors and more particularly to processes and devices for controlling the power supplied to industrial electrical furnaces, particularly in the glassmaking industry, for example, furnaces for heating of glass for the tempering and/or bending of the glass.
2. Discussion of the Background
It is important that the power supplied to the electrical resistors, particularly of a furnace, for example, intended for heating glass sheets for their tempering and/or their bending, be strictly controlled, otherwise the various glass sheets will be treated differently and will be bent and/or will be tempered differently from one another. This will result in disparities in the shapes of the glazings thus produced, as well as in their tempering state.
For these tempering and/or bending furnaces of glass sheets in particular, a great precision in the electric power dissipated is needed and it is desired in particular to have only variations of less than 1%.
Besides the advantages to be derived with respect to tempering and/or bending of the glass already mentioned, the precise control of the power dissipated would make it possible to avoid the effect of disturbances of the general electric power network and/or avoid other disturbances, and to cause the installations to operate with more regularity, which is beneficial for their service life.
Of course, there are similar controls of thyristors which could be satisfactory to control industrial electrical furnaces, but in these cases, there are then associated with each thyristor one or more electronic control and conversion circuit cards whose cost are high. When it is considered that for large installations, it is necessary to control a hundred or more thyristors, the cost that this type of installation would represent becomes clear.